Furnaces used in the smelting and converting of ferrous and non-ferrous ores and concentrates generally have a bottom wall (hearth) and vertical walls (sidewalls) comprised of refractory bricks, a structural metal shell surrounding the refractory hearth and sidewalls, and a roof or off-gas hood. Adequate compression of the furnace walls, and particularly the hearth, is critical to maximize furnace campaign life and to prevent costly and potentially catastrophic furnace failure.
During heating of the furnace to operating temperature, the individual bricks comprising the hearth and the wall refractories expand, resulting in outward expansion of the furnace. Conversely, cooling of the furnace results in contraction of the individual bricks and overall shrinking of the furnace. If the compressive forces on the hearth or the walls are insufficient, gaps may be formed between the bricks during cooling phases of the furnace operation. These gaps can be infiltrated with molten metal or other material, resulting in permanent, incremental growth of the furnace as it is repeatedly heated and cooled. This incremental expansion of the furnace, known as ratcheting, can reduce the furnace campaign life by yielding the steel shell to the point that it eventually ruptures, and/or by allowing the molten furnace contents to escape through the expanded and infiltrated joints between bricks.
Binding systems for rectangular furnaces are well known, and generally comprise regularly spaced vertical beams known as buckstays, which are held together at the top and bottom by resilient horizontal tie members extending across the furnace side walls. This binding arrangement can provide a substantially constant load on the furnace wall and hearth refractories, independent of furnace thermal expansion or contraction, thus preventing thermal ratcheting and infiltration of brick joints. However, such binding systems are not directly adaptable to use in circular furnaces.
The need for adequate compression is particularly important in circular furnaces, where the structural metal shell is subjected to large amounts of tension as the furnace hearth and wall refractories expand radially to a greater extent with each thermal cycle or ratchet. This problem can result in reduced furnace life or furnace failure by escape of molten furnace contents through infiltrated brick joints or by stretching of the furnace shell to the point of rupture, and has not yet been addressed in a satisfactory manner. One type of binding system for a circular furnace is described in U.S. Pat. No. 5,867,523 (Wasmund et al.), issued on Feb. 2, 1999. The system described by Wasmund et al. comprises a plurality of tensioning bindings resiliently connecting the segments of a structural metal shell of a circular furnace. These bindings apply a compressive force on the side walls of the furnace. The Wasmund patent is incorporated herein by reference in its entirety.
There remains a need for improved furnace binding systems for circular furnaces, and for circular furnaces in which tension in the outer metal shell can be maintained within acceptable limits while providing adequate compression of the brickwork to prevent thermal ratcheting and infiltration of the brick joints, particularly in the area of the hearth.